1. Field of the Invention:
The present invention relates to the stabilization of the viscosity of solutions of polysaccharide biopolymers at elevated temperatures.
2. Description of the Prior Art:
Heteropolysaccharides or biopolymers obtained by the fermentation of a carbohydrate by the action of bacteria of the Genus Xanthomonas, or Arthrobacter, or Alcagines, or of fungi belonging to the Genus Sclerotium, have found numerous industrial applications by virtue of their thickening and viscosity increasing properties. Polysaccharides, such as xanthan gum, are used, in particular, in the development of oil fields, in mobility control fluids in assisted recovery and also in well drilling, reconditioning and completion fluids. Xanthan gum is characterized by high viscosity at low concentrations, great insensitivity to salinity and the nature of salts and high stability under mechanical stress. However, the use of polysaccharides is limited, on the one hand, by the difficulty in preparing solutions which do not clog rock formations and, on the other, by their lack of thermal stability. For the assisted recovery of oil, the viscosity of the polymer solutions must remain stable for an extended period of time at the temperatures encountered in the reservoirs, which could be higher than 60.degree. C.. The loss of viscosity over the long term and at elevated temperatures is generally attributed to molecular degradation, with hydrolysis of the acetate, pyruvate, glucuronate radicals, release of the corresponding acids and cleavage of the glycoside bonds. The mechanism of such degradation is complex and depends upon such factors as temperature, dissolved oxygen content, pH and salinity.
Certain means have already been proposed to this art to obviate the aforesaid problems. For example, according to British Patent Specification No. 1,518,628 the stability of viscosity is improved by treatment of the aqueous liquid with a deoxidizing agent, followed by the addition of an antioxidant containing sulfur and a water-soluble oxidizable alcohol. The aqueous liquid is preferably soft water having a low salt content.
U.S. Pat. No. 4,425,246 features stabilizing heteropolysaccharide solutions containing at least 0.5% by weight. inorganic salts by deoxidation with an inert gas and/or a deoxidizing agent including sulfites, bisulfites, hydroxylamine or hydrazine dithionates, followed by heat treatment and then filtration.
In published European Patent Applications Nos. 0106665 and 0106666, saline solutions of polysaccharide substantially free of dissolved oxygen are treated at a pH at least equal to 5 with an alkali metal borohydride or a dithionate, respectively.
The aforesaid processes make it possible to stabilize the viscosity of polysaccharides in a relatively efficient manner, but they suffer from the disadvantage of frequently being incompatible with the salinity of the formation water. The addition of the deoxidizing agent may give rise to the formation of insoluble salts or hydroxides in the solution that are difficult to filter and which clog the pores of the rock formations into which said solution is injected.